1. Field of the Invention
The present invention relates to an apparatus and method for equalizing an incoming noisy digital signal for an automatic and adaptive decision feedback equalizer where a correction procedure is derived from a reference signal and a filter output signal and where the correction procedure is minimized by iteration according to the gradient procedure and is employed for setting of filter coefficients.
2. Brief Description of the Background of the Invention Including Prior Art
A superposition disturbance occurs between successive symbols in the transmission of electrical signals, which are employed for the transfer of information such as for example data. Clocked systems are employed for equalization of the symbols, where the equalization is performed in a time region. Most of the conventional, automatic and adaptive equalizers have decision feedback or are transversal filters, where variations of the equalizer transfer function are considered and evaluated. These changes in the transfer function are generated in order to obtain the best possible output signals.
In an automatic and adaptive equalizer with decision feedback the incoming noisy digital signal is fed to the filter device. The filter contains a recursive and a nonrecursive part. The filter coefficients are set by iteration according to the gradient procedure with a constant step factor. For example, for this purpose the filter output signal and a reference signal are fed to a summing device. A difference signal, that is the equalization error, is formed between the reference signal and the filter output signal and the least square error is determined. An automatic equalization is provided if ahead of the data transmission proper an arranged signal (preamble) is transmitted. For this purpose a pseudo random text or individual pulses can be employed. The reference signal can be generated by guessing (that is decision) of the equalizer output signal (estimated reference signal) or can be generated on the receiving end (ideal reference signal). In the case of adaptive setting of the equalizer no preamble is transmitted, but the reference signal is estimated.
The selection of a constant step factor in the iterative setting of the filter coefficients according to the gradient procedure is a disadvantage in such a circuit. Only inaccurate data are available in advance for the constant set step factor, since the step factor in general depends to a high degree on the characteristics of the channel. If the step factor is selected sufficiently small, then the convergence of the filter system in fact is assured, but the minimum of the equalization error is only approached very slowly. If the constant step factor is selected as too big, then the equalization setting diverges. The equalization error then deviates strongly from the minimum and the achieved equalization is insufficient.
Miloradovic et al. in U.S. Pat. No. 4,037,160 teach adjusting and readjusting an automatic connector for a data signal transmission system. The corrector serves to restore pulse-amplitude-modulated signals which have suffered linear distortions in a transmission channel. A transversal filter of the corrector receives during a first adjustment phase a reference signal sequence transmitted over the transmission channel and an identical reference signal produced at the receiving end to produce error signals which serve to adjust the transversal filter coefficients stored in the corrector. A transversal filter is a special case of a nonrecursive filter. The filter coefficients are subdivided into at least two groups and during the first phase one group of filter coefficients is calculated with a more attenuated error signal than the filter coefficients of the other group. The output signal of the transversal filter is correlated with the data signal arriving at the input of the transversal filter.